iommu: Remove iommu_register_instance interface

And also move its remaining functionality to
iommu_device_register() and 'struct iommu_device'.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Frank Rowand <frowand.list@gmail.com>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: devicetree@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Joerg Roedel <jroedel@suse.de>